Capacitive sensors used for measuring a physical parameter may comprise a mobile common electrode that forms part of an armature resiliently held between two fixed electrodes. This common electrode is capable of moving a certain distance in the direction of one or the other of the fixed electrodes as a result of an applied force, for example. In the inoperative state, the common electrode is at an approximately equal distance from the two fixed electrodes, which defines equal capacitive values for the two capacitors. When the common electrode moves, for example as the result of an applied force, the capacitive value of each capacitor varies inversely to the capacitive value of the other. The electronic interface circuit of a capacitive sensor is thus arranged to provide an output signal in the form of a voltage dependent on the variation in the capacitance of the two capacitors. This kind of capacitive sensor can be used for instance to measure acceleration values.
An electrical interface circuit is connected to the measurement sensor to process the signal from the measurement sensor. Conventional interface circuits, such as the one described in the patent U.S. Pat. No. 8,429,981 B2, use a digital-to-analogue converter (DAC) in order to provide voltage as input for pre-charging the capacitances of the capacitive measurement sensors. The resolution of this DAC is directly linked to the resolution of the digital output of the measurement circuit. For instance a 10-bit resolution output needs a 10-bit resolution DAC in a feedback circuit of the interface circuit. The conventional techniques may be very efficient for a low resolution output (e.g. up to a 10-bit resolution). However, some linearity problems inherent to the precision of the DAC appear beyond this resolution. Furthermore, in the existing solutions, a repetitive pre-charge phase is needed for pre-charging the measurement sensor capacitances. However, the disadvantage of this pre-charge phase is that no measurement signal is generated during this phase, while some energy is used to charge the capacitors.
The patent application US 2007/0247171 A1 describes an electronic interface circuit of one or two capacitors with variable capacitance. Said interface circuit includes a differential integrating amplifier with two inputs, a switching circuit for charging each capacitor by different voltages in a first phase, and to connect a fixed electrode of each capacitor to one respective input in a second phase. Each capacitor is biased by reverse different voltages in a third phase, and their fixed electrode is connected to a respective input of the amplifier in a fourth phase. A common electrode of the capacitors is connected to earth. The two output signals from the amplifier are compared in a comparator to control the switched capacitor circuit. The operation of said electronic interface circuit is relatively long to obtain a measured value at output, and the excitation of the capacitors is performed in a symmetrical manner, which are drawbacks.
The patent application US 2010/0231237 A1 describes an electronic circuit with a capacitive sensor for measuring a physical parameter. The sensor includes two capacitors mounted in differential, whose a common electrode is connected to one input of a charge transfer amplifier. An integrator is connected to the output of the charge transfer amplifier and is controlled by a dynamic comparator. The measured output of the integrator can be applied to the fixed electrodes of the capacitors through an excitation unit in a first phase of operation. In a second phase, the fixed electrodes of the capacitors are biased by two different voltages through the excitation unit. It is necessary to have several measuring cycles to obtain a final measured value at output of the electronic circuit, which is a drawback.
In the patent applications DE 10 2005 031607 A1 and US 2007/0236373 A1, it is only described a circuit for converting the capacitive signal change of a differential capacitor into digital signals. Different capacitors can be connected to an input of an integration amplifier followed by a comparator.